The present invention relates to method of inspecting reticles and an apparatus therefor, wherein in exposure process of a reticle and a mask to be used for manufacturing an LSI or a print board, foreign substance or defect on the reticle and the mask is detected before pattern on the reticle and the mask is transferred onto a wafer.
In the exposure process used during manufacturing an LSI, a chromium pattern on a thick board called a reticle is printed and transferred to a semiconductor wafer. In this process, when foreign substance and defect exist on the reticle, since the pattern cannot be transferred to the semiconductor wafer exactly, all LSI chips become a defective unit. Consequently, inspection of foreign substance and defect before the exposure is inevitable in the control of the reticle.
In addition to this, since the LSI is highly integrated in recent years and therefore the pattern becomes fine, a smaller foreign substance becomes a problem attendant upon this. Also foreign substance of a flat thin film is caused by residue of a resist during manufacturing the reticle, unfinished etching of chromium or chromium oxide for the pattern forming, and impurity melted in the reticle washing liquid and aggregated during the washing and drying. This foreign substance of the flat thin film becomes a problem, and the number of such foreign substance is apt to increase more and more.
In the prior art, an apparatus for inspecting the foreign substance and defect is proposed, as disclosed in Japanese patent application laid-open No. 65428/1984 for example, comprising means for illuminating and scanning laser light to the substrate obliquely, a first lens installed to the upper side of the substrate so as to align the illuminated point of the laser light nearly to the focal plane for collecting the scattered light of the laser light, a light blocking plate installed to a Fourier transformation plane of the first lens for blocking regular scattered light from the substrate pattern, a slit installed to the focal point of a second lens to perform inverse Fourier transformation of the scattered light from foreign substance obtained through the light blocking plate for blocking the scattered light from position other than the illumination point of the laser light on the substrate, and a light receiving unit for receiving the scattered light coming from the foreign substance through the slit.
In this proposal, paying attention to that pattern is generally constituted in the same direction or in combination of several different directions within the visual field, diffraction light by the pattern in this direction is removed by the space filter installed to the Fourier transformation plane, thereby only the reflected light from the foreign substance is emphasized and removed.
Also in the prior art, as disclosed in Japanese patent application laid-open No. 139278/1983 for example, method for comparing data detected using an illuminating and detecting optical system similar to the exposure unit with data of a standard reticle or data in the design and for detecting the defect is proposed.
This method is in that data detected using the detecting optical system is binarized and compared with the binary data of the pattern estimated from the design data.
Further, the prior art is disclosed in U.S. Pat. No. 4,595,289 or U.S. Pat. No. 4,330,205.
Among the above-mentioned prior art, Japanese patent application laid-open No. 65428/1984 is characterized in that reflected light from a foreign substance is separated by a light blocking plate from reflected light from a pattern and only the reflected light from the foreign substance is detected by the slit, and that since the foreign substance is detected by the binarization method, the detecting mechanism is simplified. On the other hand, however, since the foreign substance is detected by illumination of laser light from oblique upper direction being different from the original exposure unit, so to speak, by indirect illumination, only the reflected light from the chromium pattern of specific angle is blocked but the foreign substance from all chromium pattern cannot be discriminated.
Also in the case of detecting by indirect means as above described, foreign substance without producing actual damage (hereinafter referred to as "false alarm") also may be detected. Particularly the pattern becomes fine and the number of foreign substances producing a problem is increased but the number of foreign substances producing no actual damage is also increased, thereby the number of false alarms is increased, and check regarding whether the detected foreign substance produces a problem or not, analysis and removing of the foreign substance and other works are increased and therefore the working efficiency is significantly deteriorated.
Among the above-mentioned prior art, Japanese patent application laid-open No. 139278/1983 is characterized in that since the optical system similar to the exposure unit is provided, constitution of the optical system is simplified in comparison to the prior art. On the other hand, however, a problem exists in that the image signal processing system for comparing data is complicated in comparison to the prior art and much time is required for the inspection.
The reference data is binary image, but since detection signal must be detected by tone image of multiple values due to limitation of the resolving power of the optical system, the detection signal is binarized and compared. During the binarization, even true pattern is binarized in the shape different from the reference pattern, which causes increase of the false alarms. In order to eliminate the false alarm, since algorithm is adopted where difference of several pixels is not made defect, a problem exists in that defect as large as several pixels may be overlooked. If the pixel size is made small in order to increase the resolving power as the measure for the overlooking, a problem exists in that much time is required for the inspection.
Method disclosed in U.S. Pat. No. 4,595,289 is according to comparison inspection using dark-field illumination, but even by this method, a problem exists in that a plurality of circuit corner portions enter one pixel of the detector, and when the scattered light signal from the circuit pattern becomes larger, it is difficult to decrease influence of the alignment error. That is, since the number of the circuit pattern corners detected by one pixel is varied due to misalignment, the signal level detected by corresponding one pixel of two detection systems comparing with each other is significantly varied.
U.S. Pat. No. 4,330,205 relates to an apparatus for detecting optical defects using laser beam, but this also does not provide a simple and effective detector by the comparison detection as in the present invention.